It's a tool for fetching things and making sure what you fetch is what it's supposed to be.

For facebook in particular I'd guess it's basically just a bootstrapping step for buck2 and other basic tools like that.

Containers are obviously tempting but I think they're basically a way of shoving complexity under the bed whereas the approach that dotslash nudges you towards is slightly hormetic in that it helps you manage the complexity while also acting a slight tax on it's growth.